1. Field of the Invention
The present invention relates to an apparatus, a method, and a computer product for image processing.
2. Description of the Related Art
With development in technologies for image processing in recent years, it becomes possible to produce a copy that cannot be easily distinguished from an original. To prevent forgery of specific documents such as bank notes and marketable securities, there is an increasing demand for technologies for preventing copying or producing a perfect copy of such documents.
Under the circumstances, various technologies have been proposed for regulating copying of specific or confidential documents.
For example, Japanese Patent Application Laid-open No. 2004-274092 discloses a conventional technology for inhibiting copying of image data by adding copy prohibition information to the image data using a dot pattern. In the conventional technology, when a dot pattern that has an amount of characteristics equal to that of a copy prohibition pattern previously stored is detected from image data, it is determined that the image data is prohibited from being copied.
Another conventional technology has been proposed that uses a dot pattern as a watermark pattern. The watermark pattern is a recognizable character string or image that appears on a copy of image data, and is formed of dots having a predetermined diameter visible on the copy and dots having a predetermined diameter invisible on the copy.
However, if such a dot pattern that prohibits output of image data is superimposed on the entire image data containing content such as text data and photographic data, visibility or appearance of the content printed on the copy decreases (see FIG. 14).
Adding such dot pattern as copy prohibition information to image data is useless if the copy prohibition information is not extracted with predetermined accuracy regardless of conditions for reading and outputting the image data. Therefore, the dot pattern to be superimposed on the image data needs to have a predetermined amount of density or luminance. To use a dot pattern as a watermark pattern, it suffices that the dot pattern added to image data includes two types of dots having different diameters. In this case, it is desirable that the dot pattern to be added to the image data have the minimum density (luminance) to prevent decrease in the visibility of the content information.